1. Field of the Invention
The present invention relates to a multi-output switching power supply having a plurality of outputs.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a configuration of a conventional resonance-type multi-output switching power supply. In this multi-output switching power supply, a primary side of a transformer T1 is provided with: a full-wave rectifying circuit 2 which rectifies an AC voltage from a commercial power source 1; a smoothing capacitor C3 which is connected between output terminals of the full-wave rectifying circuit 2, and smoothes an output of the full-wave rectifying circuit 2; a first switching element Q1 and a second switching element Q2 which are made of, for example, MOSFETs and are connected in series between both ends of the smoothing capacitor C3, and to which a voltage between the both ends of the smoothing capacitor C3 is applied as a DC input voltage Vin; a control circuit 10 which controls the turning on and off of the first and second switching elements Q1 and Q2; a voltage resonance capacitor Crv which is connected in parallel with the second switching element Q2; and a series resonant circuit which is connected to both ends of the voltage resonance capacitor Crv.
The series resonant circuit is formed by connecting a primary winding P1 (a winding number N1) of the transformer T1, a reactor Lr, and a current resonance capacitor Cri in series. Note that the reactor Lr is formed of, for example, a leakage inductance between primary and secondary sides of the transformer T1.
Moreover, the secondary side of the transformer T1 is provided with: a first rectifying and smoothing circuit connected to a first secondary winding S1 (a winding number N2) which is wound so as to generate a voltage having a phase opposite to that of a voltage of the primary winding P1 of the transformer T1; and a second rectifying and smoothing circuit connected to a second secondary winding S2 (a winding number N3) which is wound so as to generate a voltage having a phase opposite to that of the voltage of the primary winding P1 of the transformer T1.
The first rectifying and smoothing circuit includes a diode D1 and a smoothing capacitor C1. The first rectifying and smoothing circuit rectifies and smoothes the voltage induced by the first secondary winding S1 of the transformer T1, and outputs the rectified and smoothed voltage, as a first output voltage Vo1, from a first output terminal. The second rectifying and smoothing circuit includes a diode D2 and a smoothing capacitor C2. The second rectifying and smoothing circuit rectifies and smoothes the voltage induced by the second secondary winding S2 of the transformer T1, and outputs the rectified and smoothed voltage, as a second output voltage Vo2, from a second output terminal.
Moreover, the multi-output switching power supply described above includes a feedback circuit 5 for feeding back a signal corresponding to the voltage generated in the secondary side of the transformer T1 to the primary side. An input side of the feedback circuit 5 is connected to the first output terminal. This feedback circuit 5 compares a voltage between both ends of the smoothing capacitor C1 with a predetermined reference voltage, and feeds back an error voltage, as a voltage error signal, to the control circuit 10 on the primary side.
The control circuit 10 performs a PWM control so as to set the first output voltage Vo1 constant by alternately turning on and off the first and second switching elements Q1 and Q2 based on the voltage error signal fed back from the feedback circuit 5. In this case, as a control signal, such a voltage as to allow a dead time of about several hundred nS is applied to each of gates of the first and second switching elements Q1 and Q2. Thus, the first and second switching elements Q1 and Q2 are alternately turned on and off without allowing the respective on periods thereof to overlap with each other.
Next, with reference to a waveform diagram shown in FIG. 2, operations of the conventional multi-output switching power supply thus configured will be described.
In FIG. 2, VQ2ds is a voltage between a drain and a source of the second switching element Q2. IQ1 is a current flowing through a drain of the first switching element Q1. IQ2 is a current flowing through the drain of the second switching element Q2. Icri is a current flowing through the current resonance capacitor Cri. Vcri is a voltage between both ends of the current resonance capacitor Cri. ID1 is a current flowing through the diode D1, and ID2 is a current flowing through the diode D2.
The control circuit 10 controls the first output voltage Vo1 by receiving the voltage error signal fed back to the primary side through the feedback circuit 5 from the first rectifying and smoothing circuit, and by thus performing the PWM control of the first switching element Q1. In this case, the first and second switching elements Q1 and Q2 are alternately turned on and off with the dead time of about several hundred nS according to the control signal from the control circuit 10, as described above.
First, in the on period (for example, time t1 to t2) of the first switching element Q1, energy is stored in the current resonance capacitor Cri through an exciting inductance of the primary winding P1 of the transformer T1 and the reactor Lr (the leakage inductance between the primary and secondary sides of the transformer T1).
Next, in the on period (for example, time t2 to t4) of the second switching element Q2, the energy stored in the current resonance capacitor Cri allows the reactor Lr and the current resonance capacitor Cri to generate a resonance current, and the energy is transmitted to the secondary side. Moreover, exciting energy of the exciting inductance of the primary winding P1 is reset.
To be more specific, in the on period of the second switching element Q2, a voltage obtained by dividing the voltage Vcri between the both ends of the current resonance capacitor Cri by the exciting inductance of the primary winding P1 and the reactor Lr is applied to the primary winding P1. Then, when the voltage applied to the primary winding P1 reaches (Vo1+Vf)×N1/N2, clamping is performed. Accordingly, the resonance current generated by the current resonance capacitor Cri and the reactor Lr flows, and the energy is transmitted to the secondary side. Thus, the current ID1 flows through the diode D1. When the voltage applied to the primary winding P1 is less than (Vo1+Vf)×N1/N2, no energy is transmitted to the secondary side of the transformer T1. Moreover, a resonance operation is performed only on the primary side by the exciting inductance of the primary winding P1 of the transformer T1, the reactor Lr and the current resonance capacitor Cri.
When an on-duty of the first switching element Q1 is Don, the voltage of the current resonance capacitor Cri is subjected to a resonance operation around about Vin×Don. Thus, the output voltage Vo1 is set to about Vin×Don×(Lp/Lri)×(Ns/Np). With respect to a change in a load, only amplitude of the voltage of the current resonance capacitor Cri is changed, and the duty is hardly changed. The duty is changed only with respect to a change in an input voltage.
Moreover, the first and second secondary windings S1 and S2 are connected to each other with the same polarity. Thus, in the on period of the second switching element Q2, while energy obtained from the first secondary winding S1 is outputted as the first output voltage Vo1, energy obtained from the second secondary winding S2 is also outputted as the second output voltage Vo2. The second output voltage Vo2 is set approximately to Vo1×N3/N2.
As described above, the second output voltage Vo2 is set to a voltage obtained by multiplying the first output voltage by a ratio of the winding number of the first secondary winding S1 to the winding number of the second secondary winding S2. Thus, if the first secondary winding S1 has a reduced number of turns, it becomes more difficult to obtain a required voltage.
FIG. 3 is a circuit diagram showing a configuration of another conventional multi-output switching power supply. This multi-output switching power supply is provided with a regulator 12 such as a dropper and a step-down chopper, in place of the second rectifying and smoothing circuit shown in FIG. 1. This regulator 12 is used to generate a second output voltage Vo2 from the first output voltage Vo1 for the purpose of stabilizing outputs. The multi-output switching power supply described above can solve a problem of cross regulation of two outputs. However, losses caused by the regulator 12 are increased, and costs and a mounting area are increased by adding components such as a switching element, choke coil and a control IC. Furthermore, occurrence of noise due to a switching regulator, such as the step-down chopper, is unavoidable.
Moreover, as a multi-output switching power supply, Japanese Patent Laid-Open Official Gazette No. 2003-259644 discloses a switching converter circuit which stabilizes two kinds of voltages by use of a single converter. This switching converter circuit includes an active snubber formed of a second switching element, stabilizes a first output by controlling the turning on and off of a first switching element, and stabilizes a second output by controlling the turning on and off of the second switching element while the first switching element is off. The switching converter circuit described above can stabilize the two kinds of outputs by use of the single converter. However, a secondary winding for obtaining the first output and a secondary winding for obtaining the second output are required to have polarities opposite to each other. Thus, two secondary windings are required.
As described above, the second output voltage Vo2 is determined by the ratio of the winding number of the first secondary winding S1 to the winding number of the second secondary winding S2. Thus, a required voltage may not be obtained. In the configuration including the regulator on the secondary side for solving the above problem, losses caused by the regulator are increased, and costs and the mounting area are increased by adding components. Furthermore, there is a problem that noise is caused by the regulator. Moreover, in the switching converter circuit disclosed in Japanese Patent Laid-Open Official Gazette No. 2003-259644, a plurality of secondary windings of a transformer are required. Thus, there is a problem that the configuration becomes complicated.